1. Field of the Invention
The present invention relates to a control system for an internal combustion engine having an exhaust gas recirculation device that recirculates exhaust gases to an intake system, and particularly to a control system that estimates an amount of oxygen in a cylinder of the engine and performs fuel injection control according to the estimated amount of oxygen.
2. Description of the Related Art
Japanese Patent Laid-open No. 2006-29171 (JP '171) discloses a conventional control system which estimates an amount of oxygen contained in the air-fuel mixture in the cylinder before combustion (in-cylinder oxygen amount) based on a detected intake air amount and an estimated amount of recirculated exhaust gases. The control system then determines a fuel injection control parameter for a fuel injector according to the estimated in-cylinder oxygen amount.
According to the control system disclosed in JP '171, the control can be based on a gas temperature TI (hereinafter referred to as “intake air temperature”) in an intake pipe. However, what affects the actual combustion characteristic of the air-fuel mixture in the cylinder is a temperature of the air-fuel mixture compressed in the cylinder. Accordingly, by only taking the intake air temperature TI into consideration, it is rather difficult to constantly maintain a stable combustion state, particularly in a so-called low temperature combustion mode or a premix combustion mode of a diesel engine.
Further, according to the above-described conventional control system, good performance cannot be obtained in the transient state of the engine. Therefore, there is a problem that the combustion noise becomes particularly large immediately after the end of the fuel cut operation, at which point, the oxygen concentration in the recirculated exhaust gases becomes relatively high.
Further, there is a problem that a torque shock occurs when shifting from an idling condition to a normal operating condition (e.g., a condition where a constant torque is generated at the rotational speed of about 2000 rpm), or vice versa, i.e., when the normal operating condition shifts to the idling condition.
Further, during the above-described transient state, the in-cylinder oxygen amount tends to be insufficient, and the combustion state may sometimes become unstable.
Further in the conventional control system disclosed in JP '171, it is necessary to increase a boost pressure in order to increase the in-cylinder oxygen amount in a high load operating condition wherein the accelerator pedal is greatly depressed, the exhaust gas recirculation is stopped, and the throttle valve is fully opened. However, since the boost pressure is in the vicinity of the maximum boost pressure, the rate of increase in the boost pressure is relatively low. Consequently, there is a problem that the in-cylinder oxygen amount becomes insufficient for the demand output, and the incremental amount of fuel is also insufficient, resulting in bad accelerating performance of the engine.